This invention relates generally to bearings. More particularly, the invention relates to bodies used for bearings, wherein the bodies comprise polycrystalline diamond and cemented metal carbide pressed at ultra high pressure and temperature. The bearings described herein are particularly useful in earth boring and like operations.
Earth boring applications typically occur in a very hostile environment. For example, harsh chemicals and extreme temperatures are common in geothermal well drilling. Furthermore, drill bits are subjected to extremely high impact forces, particularly in transitions between hard and soft rock formations. And in virtually all applications higher loads on the drill bit are desirable to achieve faster penetration rates, thereby decreasing drilling time and cost. The bearings in drilling tools are most vulnerable to the hostile drilling environment. Accordingly, much effort has recently been devoted to improving wear and impact resistance, and load capacity of materials suitable for use as a bearing.
Polycrystalline diamond (PCD) has been suggested for use as a bearing in earth boring applications. See, e.g., U.S. Pat. Nos. 4,260,203 and 4,029,368. Excessive fracturing and spalling of the PCD, on macro and microscopic scales, however, has severely limited the use of PCD as a bearing.
Because diamond has a high modulus of elasticity, PCD has relative low impact resistance. The low impact resistance of PCD, which has been identified as at least one cause of the fracturing and spalling of PCD, is particularly disadvantageous in bearing applications where the PCD is subject to high impact forces and where a high load bearing capacity is required, such as in earth boring applications.
In an attempt to circumvent the problems associated with brittle PCD, PCD products originally included a precemented carbide substrate, yielding what is termed a "composite compact". The presence of a substrate, however, causes additional problems. Because the precemented carbide substrate has a higher coefficient of thermal expansion than PCD, stresses are created when the PCD composite compact cools from the 1,300.degree. to 2,000.degree. C. temperatures at which the composite is made; the carbide substrate shrinks more than the diamond, and because the diamond layer is less elastic than the carbide substrate, these stresses often cause cracking of the diamond layer, either during cooling or during use.
Another disadvantage to having a substrate directly bonded to the PCD is that some metals will not bond to PCD. For example, steel, which has a low modulus of elasticity and would be preferred in high impact applications such as earth boring, cannot presently be bonded to PCD because ferrous materials (such as steel) catalyze the graphitization of PCD and are therefore chemically incompatible.
Yet another disadvantage of having a substrate directly bonded to the PCD is that the thickness of the PCD is limited. The limitation exists mainly because of a phenomenon called "bridging". Bridging occurs when a fine powder is pressed from multiple directions, and individual particles in the powder stack up and form arches or "bridges" so that the center of the powder does not receive the full amount of pressure. When a 1 micron diamond powder is used to make a PCD compact with is more than about 0.06 inches thick, for example, the PCD toward the center of the piece is usually not as well formed as the exterior. This condition causes cracking and chipping of the diamond layer.
Accordingly, there is a need for a PCD bearing material that is wear resistant, impact resistant, and has a high load capacity.